Synthesis Of Cobalt Phosphide/Carbon Nitride Composite Materials And Research On Photocatalytic Properties

As a kind of fuel,hydrogen has attracted much attention all over the world because of its easy access to raw materials,clean combustion products and high energy density,etc.It has been seen as a promisig method to convert solar energy into chemical hydrogen fuel that scientists synthesize photocatalytic semiconductors to produce hydrogen from water decomposition through artificial photosynthesis system.Block carbon nitride?g-C₃N₄?has low photocatalytic activity due to its small specific surface area,fewer reactive sites,high photogenerated electron hole recombination rate and low response rate to visible light.Therefore,the researchers modified g-C₃N₄ by various methods to enhance its photocatalytic performance,including morphology control,heterogeneous structure construction,metal or nonmetallic element doping,precious metal deposition,defect control and dye sensitization,etc.In recent years,photocatalytic catalyst materials based on transition metal phosphide?TMP?have been favored by researchers because of their low cost,rich content and good conductivity.The phosphate atoms in the phosphates act as bases to capture positively charged protons,thus increasing the rate of charge separation.Aiming at the photocatalytic problems of the block g-C₃N₄,we prepared porous g-C₃N₄?P-g-C₃N₄?with larger specific surface area,more reactive sites and electron transport channel by hydrothermal pretreatment and calcining with the precursors of melamine and urea,and prepared binary photocatalyst?CoP/P-g-C₃N₄?for photocatalytic hydrogen production by simple grinding with transition metal cobalt phosphide?CoP?.In addition,in order to further improve the efficiency of photocatalytic hydrogen production,we used melamine to prepare carbon nitride nanosheets?CNNS?through air thermal oxidation exfoliation method,then the heterojunction BCNDs/CNNS was constructed with boron doped carbon nitride dots?BCNDs?and CNNS through hydrothermal synthesis method,and finally ground them with CoP to prepare ternary photocatalyst?CoP/BCNDs/CNNS?.The main contents and conclusions of the study are as follows:?1?To solve the problem of small specific surface area,fewer reactive sites and higher photogenic electron hole recombination rate of the block g-C₃N₄ prepared by one-step calcinations,we pretreated the precursors of melamine and urea by hydrothermal method.Urea released ammonia as a pore-making agent in the process of water heat treatment,and also it acted as a supplementary nitrogen source.Then the dried sample was put in a muffle furnace by calcining to obtain larger specific surface area,more reactive sites and electron transfer channel P-g-C₃N₄.CoP as cocatalyst and P-g-C₃N₄ were grinded to get the CoP/P-g-C₃N₄ binary composite material.The structure and morphology of samples were characterized by X-ray diffraction?XRD?,Ultraviolet-visible diffuse spectrum?UV-vis DRS?,Transmission electron microscopy?TEM?,Fourier transform infrared spectroscopy?FT-IR?,BET and X-ray photoelectron spectroscopy?XPS?.The hydrogen generation performance of photocatalytic water decomposition was tested by using the on-line analysis and evaluation system of photocatalytic activity.The results showed that in the binary CoP/P-g-C₃N₄composite material,when the mass fraction of CoP was 2 wt%,the best photocatalytic decomposition rate of aquatic hydrogen was 321.19 mol·h^-1·g^-1,which was 89.22 times that of P-g-C₃N₄(3.60?mol·h^-1·g^-1)without any cocatalyst,and was1.15 times that of Pt/P-g-C₃N₄(278.89?mol·h^-1·g^-1).CoP,as a cocatalyst,replaced the expensive precious metal platinum,and promoted the effective separation of electron holes,which distinctly enhanced the photocatalytic activity of the catalyst.?2?To solve the problem that the above steps of preparing P-g-C₃N₄are tedious and low yield,we obtained CNNS by simple air thermal calcining method.The prepared BCNDs and CNNS were compounded by hydrothermal method to form the BCNDs/CNNS heterojunction,then the BCNDs/CNNS heterojunction was simply grounded with the cocatalyst CoP to obtain the ternary CoP/BCNDs/CNNS composite.The results showed that CoP/BCNDs/CNNS ternary composite had excellent catalytic activity in hydrogen generation of water decomposition.When the volume of BCNDs was 10 m L and the mass fraction of CoP was 3wt%,the catalytic water decomposing hydrogen rate was 1332.81?mol·h^-1·g^-1,which was 9.70 times than that of CoP/CNNS(136.98?mol·h^-1·g^-1)and 11.97 times than that of BCNDs/CNNS(111.30?mol·h^-1·g^-1),and it was approximately equivalent to that of Pt/BCNDs/CNNS(1395.58?mol·h^-1·g^-1).The block g-C₃N₄ was exfoliated into CNNS with large specific surface area by thermal oxidation exfoliation.The hydrothermal method was used to compound BCNDs and CNNS to construct heterojunction to promote electron hole separation.CoP,as a cocatalyst,further promoted electron and hole transfer,and boosted the photocatalytic activity of the catalyst.